Modern society depends upon electronic communication for many of its functions, where electronic communication may generally be divided between analog communications and digital, or discrete, communications. Digital communication presently is the predominant form of communication.
Digital communication is simply the process of exchanging information using finite sets of symbols that are represented by different types of signals. In modern practice, these signals may be electrical waveforms, for example, propagated from point to point along a controlled impedance transmission path of a printed circuit board (PCB). In other forms of modern practice, for example, digital communication utilizes a free space medium, using electromagnetic fields to propagate the information from one point to another. Still other transmission media includes an optical data path as utilized, for example, by the Synchronous Optical NETwork (SONET).
In any case, a serial communication channel is established to convey serial data from a transmitter to a receiver, whereby data timing integrity is maintained by synchronizing the relative timing between the transmitter and the receiver. That is to say, that the clock signal used by the transmitter should either be transmitted to the receiver in a separate channel, i.e., clock forwarding, or combined with the transmitted data and then sent to the receiver as a composite signal. Synchronization is achieved, therefore, when the receiver recovers the transmitted clock signal, thus establishing synchronism with the transmitter and then utilizing the recovered clock signal to latch the received data.
The use of a composite clock and data signal is generally preferred over clock forwarding for several reasons. First, the composite signal is insensitive to relative timing skews between the respective clock and data signals. Since the composite signal is subject to the same multi-path, fade, delay, reflection, and other signal degradation phenomenon, the relative effect on the data and clock signals is virtually non-existent. Second, the composite signal only requires a single channel for transmission, whereas clock forwarding requires two channels: one for the data signal; and one for the clock signal. The composite signal is then subjected to a Clock and Data Recovery (CDR) circuit at the receiving end in order to extract the respective clock and data components of the composite signal.
Basic approaches to accomplish the CDR function include, for example, a Surface Acoustic Wave (SAW) based CDR and a Phase-Lock Loop (PLL) based CDR. The SAW based CDR utilizes a high Q band-pass filter having an extremely narrow pass-band. Due to the inherent narrow band operation of the SAW filter, spectral energy relating to the clock frequency is readily available at the output of the SAW filter. After compensation of the SAW filter delay is performed, the resultant clock signal may be used to latch the received data. One advantage of using a SAW based CDR, is that very little phase jitter is introduced by the CDR, due to the passive and high Q nature of the SAW filter.
PLL based CDR is another popular method of extracting the clock and data signals from the composite signal. A phase-locked loop is utilized to phase lock to the received composite signal and to generate a clock signal that is substantially synchronized to the transmitted clock signal. Once the clock signal is generated, it can then be used to extract the data signal from the composite signal.
One drawback of both the SAW based and PLL based CDRs, however, stems from their dependency on data transitions within the composite signal. For example, if no spectral energy relating to the clock portion of the composite signal exists, then the output of the SAW filter is simply narrow band noise. Likewise, lack of signal transitions within the composite signal usually causes the phase detection component of the PLL based CDR to fail or incorrectly report phase error, thus causing the PLL to eventually drift in frequency and lose synchronization with the transmitting device.
Lack of data transitions within the composite signal may be attributed to long run lengths within the data sequence or simply a cessation of data transmission. Framed data sequences may be coded in such a way as to mitigate long run lengths such that at least a minimum transition frequency within the composite signal may be ensured. 8b/10b codes exemplify such a coding, in which 8 bits of data are encoded into a 10 bit data word, such that run lengths of no more than 5 bits and minimum transition densities are guaranteed. The 8b/10b coding scheme, however, has disadvantages of consuming the additional channel bandwidth used by the extra 2 bits and requiring encoding hardware at the transmitter and decoding hardware at the receiver. As an alternative, bit scrambling may be used to lower the Direct Current (DC) content of the transmitted signal and to increase the number of zero crossings with low transition density data so as to facilitate clock recovery. Bit scrambling, however, does not totally preclude the possibility of a long stream of data being represented as a very long string of transition-less data, thus creating potential problems in both the SAW filter and PLL based CDRs.
An apparatus and method that addresses the aforementioned problems, as well as other related problems, are therefore desirable.